1. Field of Invention
The present invention relates to an automatic focusing method and device, and more particularly to an automatic focusing method and device in a high noise environment.
2. Related Art
With the development of digital cameras, photographing has been no longer a costly consumption. A user can take a desired image at will to record a moment or scenario worth memorizing. When photographing, the subject is generally focused in order to emphasize the subject. In other words, the focus distance is aligned to the object to be photographed. Therefore, most cameras have the automatic focusing function.
Referring to FIG. 1a, in the prior art of focusing, corresponding images are pre-photographed 10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h at different object distances. Then a high-frequency signal in a focusing frame for each pre-photographed image is calculated, and a relation between the high-frequency signals and the corresponding object distances as shown in FIG. 1b is derived. The optimum focusing position is determined by using the conventional curve fitting technology. However, when photographing in an environment with low luminance, a circuit is required to amplify the signal so as to obtain an image that has been properly exposed. However, noises in the image are also amplified, so that both real signals and noises are interspersed in high-frequency signals and failure of focusing often arises in severe cases. As shown in FIG. 1b, with the inferences from noises, there is often no apparent global maximum in the relation between the high-frequency signals and the corresponding object distances. As such, an automatic focusing procedure fails to determine the optimum focusing position or determines an erroneous focusing position.
In such case, in order to improve the success ratio of focusing, one of the approaches is to reduce noises in the system. For example, elements or processes with high quality are employed, or more layers are laid out in the printed circuit board. However, no matter how the noises are reduced, they still get amplified with a high gain, not to mention the increased costs in doing so. Another approach is to develop a more accurate de-noising algorithm. However, the de-noising capability thereof is limited anyway, and fabricating a precise algorithm into a circuit is rather costly. Therefore, it is still the objective for many researchers to improve the success ratio of focusing in a high-noise environment with a low cost.